Fig. 1. (a) Refractive index profile of the GRIN fiber with 150 μm core diameter. (b) Reflection spectra of the FBGs. (c) Experimental configuration of the GRIN fiber-based Raman fiber oscillator. The right-hand inset depicts the fusion and beam transfer details between the output fiber of the combiner and the GRIN fiber.

Fig. 2. Output power and spectrum evolution of the GRIN fiber-based RFL. (a) Output power evolution of the RFL. With maximum pump power injection of 2494 W, the signal power reaches 1780 W with the undepleted pump power of 340 W, corresponding to an optical-to-optical conversion efficiency of 71.4%. (b) Output spectrum evolution of the RFL. The FWHM linewidth of the Stokes signal broadens from 0.68 to 1.89 nm during power scaling, as a result of the nonlinear effects.

Fig. 3. Output beam characteristics of the Raman fiber laser at various powers. (a) The measured and reconstructed beam profiles of the signal laser at various power levels. (b) The and BE factor of signal light at various power levels. (c) The fundamental mode weight of signal light at various power levels.

Fig. 4. Mode characteristics and temporal dynamics under different output power levels. (a) Temporal variation in the content of the fundamental mode under various power levels. (b) Mean value of the fundamental mode content. (c) Time-domain signal of beam intensity. (d) Standard deviation and coefficient of variation.

Fig. 5. Fourier spectrum of the signal light’s temporal intensity at different power levels.